The present invention relates to a magneto-optical recording medium and a method of reading-out the same.
Magneto-optical recording media are popularly used as a high-density and low-cost rewritable information recording medium. Especially, those having a recording layer composed of an amorphous alloy of a rare earth metal and a transition metal, show very excellent recording characteristics.
The magneto-optical disc is a recording medium with a very large capacity, but further large capacity storage is required with the advance of the increasing amount of information in our society.
The recording density of an optical disc is usually determined by the size of the readout beam spot. Since the spot size can be lessened by shortening the laser wavelength, studies are being made for shortening the laser wavelength, but great difficulties are involved in attaining this object.
On the other hand, various attempts such as a super resolution technique have been made in recent years for obtaining a higher resolution than determined by laser wavelength.
One of the noticeable proposals is a "magnetically induced super resolution system" (hereinafter referred to as MSR system) which makes use of the exchange coupling force working between the multiple layers in a magneto-optical disc. In one form of this system, there is used a recording medium having an exchange-coupled magnetic layer comprising a readout layer with small coercive force, a cut-off (switching) layer with a low Curie temperature and a memory layer with a high Curie temperature and a large coercive force. When the medium is heated by readout beams while applying a readout magnetic field, the exchange coupling is interrupted at the high-temperature region of the medium. Since the readout layer per se is small in coercive force, the magnetization is oriented in the direction of readout magnetic field in the high-temperature region, thereby erasing the recording bits. Consequently, the low-temperature region alone is read out. Thus, since the readout area is actually narrowed, there is obtained the same effect as when the readout beams are off, allowing readout of high-density recording bits.
The erased recording bits are revived since the recording bits are transferred from the memory layer when the medium temperature lowers to restore exchange coupling. This system is called "front aperture detection system (FAD system)" as the signal is detected in front of the readout beam spot.
A drawback to this FAD system is that a readout magnetic field (Hr) is required for readout. Usually a readout magnetic field of not less than 24,000 A/m is required, and since the readout is conducted in the presence of such readout magnetic field of not less than 24,000 A/m, the bits recorded in the memory layer tend to become unstable.
Also, it is liable that a stronger magnetic field than necessary magnetic field for recording would be required for readout. This becomes a serious problem in the attempts at miniaturizing the magnetic head and simplifying of the apparatus. Especially, in the magnetic field modulation recording, the recording magnetic field is mostly less than 10,000 A/m and the application of the readout magnetic field gives rise to serious problems.
Thus, there has been strong demand for the realization of a super resolution-type magneto-optical recording medium capable of providing a high C/N (Carrier/Noise) ratio with no need of applying a readout magnetic field, and a method for readout of such recording medium.
The realization of a magneto-optical recording medium having a memory layer which possesses antithetical properties of being capable of generating a high magneto-static field (properties as a magnet) and being high in perpendicular magnetic anisotropy, according to which there is no possibility that the perpendicular magnetic anisotropy of the memory layer be reduced by high magnetization for reversing the magnetization direction of the readout layer in the magneto-static field to cause slanting of the magnetization direction, nor is the possibility of producing a minute reversal region in the magnetic domain in the memory layer.
Another means for realizing the large capacity storage is use of a pulse width modulation (PWM) recording system. For optical disc recording, there have been conventionally used a pulse position modulation (PPM) recording system in which the pulse position interval is detected, but according to the PWM recording system, the pulse end interval is detected and approximately 1.5 times mass capacity storage is realized.
A drawback to the PWM recording system is difficulty in making accurate detection of pulse ends. There are two methods for pulse end detection, for example, a method comprising setting a certain slice level and detecting a signal as a pulse end when the signal crosses that level; other method comprising second order-differentiating each signal.
The detection by the slice level system is mostly employed for optical disc recording because the detection by the second order differentiation system suffers a drop of the S/N (Signal/Noise) ratio of signals due to differentiation.
The slice level system, however, is at a disadvantage in that it is sensitive to level variation of the whole signals.
Also, when a PPM system disc is read out with the same drive, it is necessary to use a readout signal detection system in a dual way.
Further, the conventional PPM recording has involved the trouble of differentiating signals for peak detection.
As a result of intensive studies for overcoming the above related art problems, it has been found that by conducting the readout of a magneto-optical recording medium by reversing a sub-lattice magnetization direction of a readout layer at a high-temperature region by heating the magneto-optical recording medium with readout beams without applying any magnetic field,
the magneto-optical recording medium comprising a substrate and an exchange-coupled magnetic layer comprising at least the readout layer, a cut-off (switching) layer and a memory layer disposed on a substrate in order, PA1 wherein the Curie temperature (T.sub.c1) of the said readout layer, the Curie temperature (T.sub.c2) the said cut-off layer and the Curie temperature (T.sub.c3) of the said memory layer satisfy the following relations: EQU T.sub.c1 &gt;T.sub.c2 .gtoreq.50.degree. C. (1) EQU T.sub.c3 &gt;T.sub.c2 ( 2) PA1 it is possible to obtain a super resolution effect and a high C/N ratio with no need of using a readout magnetic field unlike the conventional systems using a mask. The present invention has been attained on the basis of these findings. PA1 the said magnetic layer having the properties that when the said magnetic layer is heated by irradiation of readout beams for reading out information, a sub-lattice magnetization direction of at least the layer concerned with readout in the said magnetic layer at the high-temperature region is reversed relative to the magnetization direction at the low temperature of the said region, and when the temperature of the said magnetic layer lowers after passage of the readout beams, the magnetization direction of the sub-lattice magnetization is restored. PA1 the Curie temperatures of the said readout layer, cut-off layer and memory layer satisfying the following relations: EQU T.sub.c1 &gt;T.sub.c2 .gtoreq.50.degree. C. (1) EQU T.sub.c3 &gt;T.sub.c2 ( 2) PA1 the said magnetic layer having the properties that when the magnetic layer is heated to a temperature close to T.sub.c2 or higher by readout beams, an exchange coupling force between the memory layer and the readout layer is decreased or becomes nil and a sub-lattice magnetization of at least the magnetic layer concerned with readout in a high-temperature region heated to T.sub.c2 or higher is reversed relative to the magnetization direction at the low temperature of the said region, and when the temperature of the magnetic layer lowers after passage of readout beams, the magnetization direction of the sub-lattice magnetization is restored. PA1 the Curie temperatures of the said readout layer, cut-off layer and memory layer satisfying the following relations: EQU T.sub.c1 &gt;T.sub.c2 .gtoreq.50.degree. C. (1) EQU T.sub.c3 &gt;T.sub.c2 ( 2) PA1 wherein T.sub.c1 represents Curie temperature of the readout layer, T.sub.c2 represents Curie temperature of the cut-off layer and T.sub.c3 represents Curie temperature of the memory layer; and PA1 the said magnetic layer having the properties that when the magnetic layer is heated to a temperature close to T.sub.c2 or higher by irradiation of readout beams, an exchange coupling force between the memory layer and the readout layer is decreased or becomes to nil and consequently the sub-lattice magnetization direction of the readout layer at a high-temperature region is reversed relative to the magnetization direction at the low temperature of the said region, PA1 that, at T.sub.c2 when the magnetization of the rare earth metal is dominant in the readout layer, the magnetization of the transition metal is dominant in the memory layer, or when the magnetization of the transition metal is dominant in the readout layer, then magnetization of the rare earth metal is dominant in the memory layer, and PA1 that the magnetization of the memory layer is not less than 80 emu/cc at T.sub.c2 and not more than 300 emu/cc at room temperature, and the magnetization of the readout layer is not less than 150 emu/cc at T.sub.c2 and not more than 500 emu/cc at room temperature. PA1 the Curie temperatures of the said readout layer, cut-off layer, bias layer and memory layer satisfying the following relations: EQU T.sub.c1 &gt;T.sub.c2 .gtoreq.50.degree. C. (1) EQU T.sub.c3 &gt;T.sub.c2 ( 2) EQU T.sub.c4 &gt;T.sub.c2 ( 3) PA1 wherein T.sub.c1 represents Curie temperature of the readout layer, T.sub.c2 represents Curie temperature of the cut-off layer, T.sub.c3 represents Curie temperature of the memory layer and T.sub.c4 represents Curie temperature of the bias layer; and PA1 the said magnetic layer having the properties that when the magnetic layer is heated to a temperature close to T.sub.c2 or higher by readout beams, an exchange coupling force between the memory layer and the readout layer is decreased or becomes to nil and consequently the sub-lattice magnetization direction of the readout layer at the high-temperature region is reversed relative to the magnetization direction at the low temperature of the said region, and PA1 that, at T.sub.c2 the bias layer is exchanged-coupled with the memory layer and has a higher magnetization than that of the memory layer. PA1 the Curie temperatures of said readout layer, cut-off layer and memory layer satisfying the following relations: EQU T.sub.c1 &gt;T.sub.c2 .gtoreq.50.degree. C. (1) EQU T.sub.c3 &gt;T.sub.c2 ( 2) PA1 wherein T.sub.c1 represents Curie temperature of the readout layer, T.sub.c2 represents Curie temperature of the cut-off layer, and T.sub.c3 represents Curie temperature of the memory layer; PA1 said magnetic layer having the properties that when the magnetic layer is heated to a temperature close to T.sub.c2 or higher by readout beams, an exchange coupling force between the memory layer and the readout layer is decreased or becomes to nil and a sub-lattice magnetization of at least the magnetic layer concerned with readout at the high-temperature region heated to a temperature close to T.sub.c2 or higher is reversed relative to the magnetization direction at the low temperature of said region, and when the temperature of the magnetic layer lowers after passage of the readout beams, the sub-lattice magnetization direction is restored; PA1 the magnetization of the rare earth metal in the readout layer being dominant at T.sub.c2 and the magnetization of the transition metal in the memory layer being dominant at T.sub.c2, or the magnetization of the transition metal in the readout layer being dominant at T.sub.c2 and the magnetization of the rare earth metal in the memory layer being dominant at T.sub.c2 ; PA1 the readout layer having a coercive force of not less than 2,000 A/m at T.sub.c2, a perpendicular magnetic anisotropy of 2.times.10.sup.-5 to 8.times.10.sup.6 erg/cc at T.sub.c2, a magnetization of not less than 100 emu/cc at T.sub.c2 and a magnetization not more than 500 erg/cc at room temperature; and PA1 the memory layer having a magnetization of not less than 80 emu/cc at T.sub.c2 and a magnetization of not more than 300 emu/cc at room temperature.
and the said recording medium has the specific properties that when the said magnetic layers are heated to a temperature close to T.sub.c2 or higher by readout beams, the exchange coupling force between the memory layer and the readout layer is decreased to or becomes nil and the sub-lattice magnetization direction of the readout layer at the high-temperature region heated to a temperature close to T.sub.c2 or higher is reversed relative to the magnetization direction at the low (pre-irradiation) temperature of the said region, and when the temperature of the magnetic layer lowers after passage of the readout beams, the original magnetization direction is restored,